The present invention relates to methods and apparatus for reproducing and after-recording digital data in tracks on a magnetic tape.
Digital VTRs for digitizing and recording color video signals on magnetic tape are in commercial use. These digital VTRs are categorized as "D1" format component VTRs and "D2" format composite digital VTRs, which are employed commonly in broadcasting stations.
In the first-developed D1 format component VTR, a luminance signal and first and second color difference signals are converted to digital from analog form using sampling frequencies of 13.5 MHz and 6.75 MHz, respectively. After these signals are appropriately processed, they are recorded on a magnetic tape. Since the ratio of the component sampling frequencies is (4:2:2), the D1 format is also referred as a (4:2:2) system.
In the later-developed D2 format digital VTR, composite color video signals are converted to digital from analog form with a sampling frequency four times the frequency fsc of the color subcarrier signal. After these signals are appropriately processed, they are recorded on magnetic tape.
Since the D1 and D2 digital VTRs are designed principally for use in broadcasting stations, the quality of the pictures produced thereby is accorded the highest priority. Accordingly, the digital color video signal, digitized at eight bits per sample, is not substantially compressed as recorded on magnetic tape. The consequently large amount of data per field is not easily accommodated in a single track as the influence of improper linearity in the tracks can substantially degrade system operation. Accordingly, the D1 format digital VTR employs a segmenting technique wherein ten tracks are employed to record each field of an NTSC-type signal, while twelve tracks are employed to record each field of a PAL-type signal. This consequently shortens the length of each track thus to reduce the influence of improper linearity even where track pitch is narrow.
Digital VTRs record digital audio signals as well as tracking pilot signals in the tracks, in addition to the digital image signal. The D1 format digital VTR records audio data in the center of each track, while recording time code and tracking control signals in the longitudinal direction of the tape. The D2 format VTR records audio data at opposite ends of each track and, like the D1 device, records time code and tracking control signals in the longitudinal direction of the tape.
In both the recording and reproducing modes of these devices, data errors sometimes occur. Thus, the digital image signal, as well as the digital audio signal and a subcode signal recorded therewith are encoded to provide error correction codes. To do so, the data is arranged in matrix format and product codes are formed therefrom, such that line codes are formed using the rows of the matrix and column codes are formed using the columns thereof. It will be seen, therefore, that each data symbol is included in two error correction code sequences when the product code is thus formed. Thus, a highly effective error correction capability can be achieved.
In conventional digital VTRs, fixed heads are utilized for recording and reproducing the tracking and time code signals. Consequently, the tape transport mechanism is fairly complex, thus tending to degrade the reliability of the tape path. It is, therefore, preferable to record the tracking and additional control data in the tracks, so that fixed heads may be eliminated.
In each track, respective recording section are provided for the digital image and audio signals, as well as for the subcode. Accordingly, after-recording requires precise control to ensure that the data is recorded in the desired section of each track. One control technique employs a detection signal synchronized with the phase of the rotary head drum to define the after-recording area. However, this technique requires greater mechanical accuracy of the magnetic or optical detectors employed than may be achieved. Consequently, the use of this control technique can result in the erasure of the previously recorded data.